Sea anemones are colorful coelenterates (related to corals, jellyfish, and Portuguese man-of-war) with stinging cells (nematocysts). Over 1,000 species live within the tidal zones of tropical, subtropical, and temperate waters. They range in size from a few millimeters to more than 0.5 meters (1.7 feet).

Sea anemone stings typically occur when they are accidentally brushed up against or purposely touched by the unknowing. Initial symptoms vary from a prickly sensation to severe burning at the wound site. The pain increases in intensity and may extend proximally into local lymph nodes. After a few hours, the pain lessens, but a residual ache or itch may last for weeks.

The skin reaction varies by sea anemone species. The venom of some species produces painful urticarial lesions; others induce erythema and edema. Some lesions may eventually blister, and in severe cases, necrosis and ulceration may result. Secondary infection is possible. A typical skin reaction is an area with central pallor surrounded by a halo of erythema and petechial hemorrhage.

Systemic symptoms may be reported and include fever, chills, malaise, nausea, vomiting, abdominal pain, headache, delirium, and muscle spasms. Shock, fulminant liver failure, renal failure, and respiratory distress have been reported.